It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene laminated to cellulose photographic grade paper for use as a reflective receiver for thermal dye transfer imaging process. In this invention, low density polyethylene is melt extrusion coated onto the backside of the reflective receiver to balance the reflective receiver for curl, provide waterproofing to the paper, and provide the proper backside roughness for printer transport.
In the formation photographic papers, where an emulsion layer containing gel is coated onto the base paper that has been extrusion coated with low density polyethylene, there is a need to provide a base paper with improved resistance to curl. When the relative humidity is greater than 50% or less than 20%, as is common in the storage of photographic images, the curl of photographic paper interferes with the viewing of images. A solution to the photographic curl problem has been proposed in U.S. application Ser. No. 08/864,228 filed May 23, 1997, U.S. Pat. No. 5,902,720. In this invention, a mechanism to reduce curl in relative humidity greater than 50% or less than 20% is accomplished by applying a biaxially oriented polyolefin sheet to the backside of the paper base to balance the forces caused by the expansion and contraction of the emulsion layer in a relative humidity environment greater than 50% or less than 20%.
While the invention in U.S. application Ser. No. 08/864,228 filed May 23, 1997 does significantly improve the humidity curl of photographic paper, the typical surface roughness of the biaxially oriented sheets described in the invention that can be applied to the backside of the paper are smooth, with an roughness average or R.sub.a less than 0.23 .mu.m. As the photographic images are processed in photoprocessing equipment (photographic printers, photographic processors, and photographic finishers), the photographic paper must be transported through many different types of equipment. In the formation of color paper it is known that the backside of the color paper is made sufficiently rough by casting polyethylene against a rough chilled roll. Photographic papers manufactured with the required backside roughness are very efficiently transported though photoprocessing equipment. Photographic papers with backside roughness less than 0.30 .mu.m cannot be efficiently transported in the photoprocessing equipment, as many transport problems will occur. Transport problems such as scratching, machine jams, and poor print stacking will begin to occur with backside roughness less than 0.30 .mu.m. It would be desirable if a backside surface could be formed with the strength properties to control curl and a surface roughness greater than 0.30 .mu.m to allow for efficient photoprocessing.
Photographic papers that are smooth on the backside will tend to stick together as the smooth backside of the print is in contact with the smooth image layer as is the case when photographic prints in the final image format are stacked for efficient storage. There remains a need for photographic papers that will not block or stick together as prints are stored.
In the final image format, it is common for consumers to write personal information on the backside of the images with pens, pencil,s and other writing instruments. The ability of the backside of the image to accept post processing writing is referred to as writability. Writability of images is a desired consumer feature. Photographic papers that are smooth on the backside are more difficult to write on than photographic papers that are rough. Further, during the photofinishing process, it is common for critical information such as processing date and time to be printed on the backside of photographic paper. The printing of information during photofinishing is referred to as back marking. Poor back marking of images results in a loss of critical information. In U.S. application Ser. No. 08/998,357 filed Dec. 24, 1997, backside roughness of a biaxially oriented polymer sheet is described as being necessary for efficient transport through photofinishing equipment. While the invention does provide the required backside roughness for efficient conveyance in manufacturing and photoprocessing, the invention does not provide a significant improvement over prior art materials for photofinishing back marking and consumer writability. There remains a need for photographic papers that are sufficiently rough to allow for efficient transport though photofinishing equipment and also allow for photofinishing back marking and consumer writability.
During the manufacturing process for photographic papers, it is a requirement that silver halide emulsion coated paper be handled and transported in roll form. In roll form, the backside of the photographic paper is in contact with the silver halide image forming layer. If the roughness of the backside exceeds 2.54 .mu.m, the image forming layer would begin to become embossed with the surface roughness pattern while in the roll form. Any customer perceived embossing of the image forming layer will significantly decrease the commercial value of the image forming layer. Furthermore, silver halide emulsions tend to be pressure sensitive. A sufficiently rough backside, in roll form, would begin to also destroy the commercial value of the image forming layer by developing the silver emulsion with pressure from the surface roughness of the backside. There remains a need for a photographic paper that has a backside roughness such that photographic paper can be conveniently wound and stored in roll format without damage but also allow writability and transport during photoprocessing.